A variety of chemical conversion coatings for aluminum are known to the art. These conversion coatings provide a corrosion resistant outer layer to the metal while often simultaneously providing improved paint or other organic coating adhesion. Conversion coatings may be applied by a "no-rinse" process in which the metal surface to be coated is cleaned and the conversion coating is dipped, sprayed or rolled on, or they may be applied as one or more coats which are subsequently rinsed from the metal surface.
Many conversion coatings are chromate-based compositions. Recently, chromate-free conversion coatings have also been developed. These coatings are particularly useful for applications, such as coating aluminum food or beverage cans, in which it is particularly desirable to avoid potentially toxic chromates. Chromate-free conversion coatings typically employ a Group IVA metal such as titanium, zirconium or halfnium, a source of fluoride ion and nitric acid for pH adjustment. These chromate-free conversion coatings are substantially clear and prevent the blackening that normally occurs when aluminum is boiled in water during pasteurization.
For example, U.S. Pat. No. 3,964,936 to Das discloses the use of zirconium, fluoride, nitric acid and boron to produce a conversion coating for aluminum. U.S. Pat. No. 4,148,670 to Kelly discloses a conversion coating comprising zirconium, fluoride and phosphate. U.S. Pat. No. 4,273,592 to Kelly discloses a coating comprising zirconium, fluoride and a C.sub.1-7 polyhydroxy compound, wherein the composition is essentially free of phosphate and boron. U.S. Pat. No. 4,277,292 to Tupper discloses a coating comprising zirconium, fluoride and a soluble vegetable tannin.
U.S. Pat. No. 4,338,140 to Reghi discloses a conversion coating comprising zirconium, fluoride, vegetable tannin and phosphate, and optionally including a sequestering agent to complex hard water salts such as calcium, magnesium and iron. U.S. Pat. No. 4,470,853 to Das et al. discloses a coating comprising zirconium, fluoride, vegetable tannin, phosphate and zinc. U.S. Pat. No. 4,786,336 to Schoener et al. discloses a coating comprising zirconium, fluoride and a dissolved silicate, while U.S. Pat. No. 4,992,116 to Hallman discloses a conversion coating comprising a fluoroacid of zirconium and a polyalkenyl phenol.
It can be seen from the above that the compositions of the prior art have not combined in high concentrations (up to the respective solubility limits) Group IA metals such as potassium with Group IVA metals such as zirconium to provide hydrophilic coatings.
It should further be noted that the conversion coatings of the prior art have not proven particularly effective for certain applications. For example, aluminum finstock used for heat exchange units, such as evaporators, has not been effectively treated using known chromate-free coatings. A need therefore exists for improved conversion coatings for providing a hydrophilic surface to aluminum finstock. The present invention addresses that need.